Preheat oven to 350° F and line muffin tin with cupcake liners.
In a large bowl, or bowl of a stand mixer, whisk together flour, baking powder, baking soda, salt, pumpkin pie spice, and brown sugar.
Add eggs, pumpkin puree, vegetable oil, milk, and vanilla and stir to combine.
Using a cupcake scoop or ice cream scoop, scoop batter into lined muffin tin; filling only until 2/3 full.
Bake at 350° F for 16-18 minutes or until toothpick in center comes out clean and cupcakes are done. Remove from oven and cool on cooling rack.
In the bowl of a stand mixer, or using electric beaters, beat together butter, marshmallow fluff, and vanilla until well incorporated. Add the powdered sugar and mix slowly until incorporated into frosting then turn mixer to high and beat for additional minute until stiff.